1. Field of the Invention
The present invention relates to a transflective liquid crystal display device for displaying images by use of a backlight under dark conditions and by use of reflection of external light under well-lit conditions.
2. Description of the Prior Art
Liquid crystal display devices are advantageous in that they are thin and light, as well as have low power consumption characteristics owing to its low-voltage drive capability, and are therefore widely used in various electronic devices. In particular, active matrix liquid crystal display devices including thin film transistors (TFTs) provided in respective picture elements as switching elements also exhibit excellent display quality equivalent to cathode-ray tubes (CRTs). Accordingly, they are widely used for televisions, displays for personal computers or the like.
In general, a liquid crystal display device includes two substrates placed to face each other, and liquid crystals sealed between the substrates. TFTs, picture element electrodes and the like are formed on one of the substrates, while color filters, a common electrode and the like are formed on the other substrate. The substrate on which the TFTs, the picture element electrodes, and the like are formed will be hereinafter referred to as a TFT substrate, and the substrate to be placed to face the TFT substrate will be hereinafter referred to as an opposing substrate. Furthermore, a structure formed by sealing the liquid crystal between the TFT substrate and the opposing substrate will be hereinafter referred to as a liquid crystal display panel.
A liquid crystal display device includes a transmissive liquid crystal display device, a reflective liquid crystal display device and a transflective liquid crystal display device. The transmissive liquid crystal display device has a backlight as a light source and displays images by use of light which passes through a liquid crystal panel. The reflective liquid crystal display device displays images by use of reflection of external light (natural light or lamplight). The transflective liquid crystal display device displays images by use of a backlight under dark conditions and by use of reflection of external light under well-lit conditions.
FIG. 1 is a perspective view showing the configuration of a conventional transflective liquid crystal display device, FIG. 2 is a plan view showing one picture element of the same transflective liquid crystal display device, and FIG. 3 is a cross-sectional view taken along the I-I line in FIG. 2. Note that, here, a description will be given of a VA (vertically aligned) mode transflective liquid crystal display device employing vertical alignment-type liquid crystals (liquid crystals having negative dielectric anisotropy).
As shown in FIG. 1, the transflective liquid crystal display device includes a liquid crystal panel 5, a drive circuit board 6 which is connected to the liquid crystal panel 5 and supplies driving signals (data signals and gate signals), a backlight unit 7 placed on one surface side of the liquid crystal panel 5 (under the liquid crystal panel 5 in FIG. 1), and a pair of circularly polarizing plates (not shown) placed so as to sandwich the liquid crystal panel 5.
As shown in FIG. 3, the liquid crystal panel 5 includes a TFT substrate 10, an opposing substrate 30, and a liquid crystal layer 40 formed of liquid crystals sealed between the substrates. The liquid crystal panel 5 is driven by integrated circuits (ICs) mounted on the drive circuit board 6 and by a driving circuit constituted of ICs 5a and 5b mounted on the periphery of the liquid crystal panel 5.
As shown in FIG. 2, gate bus lines 12 extending in the horizontal direction (X direction) and data bus lines 15 extending in the vertical direction (Y direction) are formed in a display unit of the TFT substrate 10. Each of the rectangular regions defined by the gate bus lines 12 and the data bus lines 15 constitutes a picture element region. In addition, the TFT substrate 10 is provided with auxiliary capacitor bus lines 13 which are formed in parallel with the gate bus lines 12 and crosses the picture element regions.
A TFT 16, an auxiliary capacitor electrode 18, a reflective electrode 20 and a transparent electrode 21 are formed in each picture element region. As for the reflective electrode 20, at least the surface thereof is made of metal having high reflectance such as aluminum (Al). The transparent electrode 21 is made of a transparent conductive material such as indium tin oxide (ITO), for example. The region in which the reflective electrode 20 is formed is referred to as a reflective region, and the region in which the transparent electrode 21 is formed is referred to as a transmissive region.
The TFT 16 uses a part of the gate bus line 12 as a gate electrode. A drain electrode 16d and a source electrode 16s are placed to face each other across the gate bus line 12. The drain electrode 16d is connected to the data bus line 15. Further, the source electrode 16s is connected to a pad 17a and the auxiliary capacitor electrode 18 via a wiring 17. Furthermore, the source electrode 16s is electrically connected to the reflective electrode 20 and the transparent electrode 21 via contact holes 19a and 19b. 
Hereinafter, the layered structure of the TFT substrate 10 and the opposing substrate 30 will be described with reference to FIG. 3.
The gate bus line 12 made of metal film such as a chrome (Cr) film and a laminated film of aluminum (Al)-Titanium (Ti), and the auxiliary capacitor bus line 13 are formed on a glass substrate 11 which is the base for the TFT substrate 10. In addition, a first insulating film (gate insulating film) 14 made of SiO2, SiN or the like is formed on the glass substrate 11, gate bus line 12 and the auxiliary capacitor bus line 13.
A semiconductor film 16a to be an active layer of the TFT 16 is formed on a predetermined region of the first insulating film 14. Further, a channel protection film 16b made of SiN is formed on the region of the semiconductor film 16a to be a channel. The source electrode 16s and the drain electrode 16d of the TFT 16 are formed with the channel protection film 16b interposed. The drain electrode 16d is connected to the data bus line 15 formed on the first insulating film 14. The source electrode 16s is connected to the pad 17a and the auxiliary capacitor electrode 18 via the wiring 17 formed on the first insulating film 14. The auxiliary capacitor electrode 18 is formed on a position facing the auxiliary capacitor bus line 13 with the first insulating film 14 interposed therebetween, thereby constituting an auxiliary capacitor together with the auxiliary capacitor bus line 13 and the first insulating film 14.
The TFT 16, the data bus line 15, the wiring 17, the auxiliary capacitor electrode 18 and the like are all covered with a second insulating film 19. The reflective electrode 20 and the transparent electrode 21 are formed on the second insulating film 19. The reflective electrode 20 is electrically connected to the pad 17a via the contact hole 19a. The transparent electrode 21 is electrically connected to the auxiliary capacitor electrode 18 via the contact hole 19b. The surfaces of the reflective electrode 20 and the transparent electrode 21 are covered with a vertical alignment film 22 made of polyimide or the like.
Meanwhile, a black matrix (light blocking film) 32 and a color filter 33 are formed on a glass substrate (lower surface in FIG. 3) which is the base for the opposing substrate 30. The black matrix 32 is formed of, for example, a light blocking material such as Cr, and is placed on positions facing the regions on the TFT substrate 10, where the gate bus line 12, the data bus line 15, the auxiliary capacitor electrode 13 and the TFT 16 are to be formed.
The color filter 33 is classified into three types of red (R), green (G), and blue (B), each of which selectively allows light having a predetermined wavelength range to pass through. The color filter 33 of any one color is placed in each picture element, and three picture elements of red (R), green (G), and blue (B) which are adjacently placed constitute one pixel, thereby making it possible to display various colors.
A common electrode 34 made of a transparent conductive material is formed on the color filter 33 (lower surface in FIG. 3) so as to face both the reflective electrode 20 and the transparent electrode 21 of each picture element. The surface of the common electrode 34 is covered with a vertical alignment film 35 made of polyimide or the like.
In the transflective liquid crystal display device constituted as described above, when a voltage is not applied between the common electrode 34 and both of the reflective electrode 20 and the transparent electrode 21, the liquid crystal molecules in the liquid crystal layer 40 are aligned substantially perpendicular to the substrate surfaces. In this case, in the transmissive region, the light emitted from the backlight unit 7 passes through both the circularly polarizing plate placed below the panel and the transparent electrode 21, and then enters the liquid crystal layer 40 and passes through the liquid crystal layer 40 without being changed its polarization direction. Thereafter, the light is blocked by the circularly polarizing plate placed above the panel. Specifically, black is displayed in this case. Moreover, in the reflective region, the light, which comes from above the panel passes through the circularly polarizing plate and enters the liquid crystal layer 40, is reflected by the reflective electrode 20 to travel in the upward direction, and is then blocked by the circularly polarizing plate placed above the panel. Accordingly, black is also displayed in the reflective region.
When a voltage which is higher than the specific voltage (threshold voltage) is applied between the common electrode 34 and both of the transparent electrode 20 and the reflective electrode 21, the liquid crystal molecules in the liquid crystal layer 40 are aligned in an oblique direction with respect to the substrate surfaces. In this way, the light emitted from the backlight unit 7 passes through both the circularly polarizing plate placed below the panel and the transparent electrode 21, and then enters the liquid crystal layer 40. In the liquid crystal layer 40, the polarization direction of the light is changed, and thereby the light can pass through the circularly polarizing plate placed above the panel. Specifically, a bright color is displayed in the transmissive region. In a similar way, also in the reflective region, as for the light, which comes from above the liquid crystal panel, passes through the circularly polarizing plate to enter the liquid crystal layer 40, and is reflected by the reflective electrode 20 to travel in the upward direction, the polarization direction of the light is changed as the light passes through the liquid crystal layer 40 and thereby the light can pass through the circularly polarizing plate placed above the panel.
It becomes possible to control the amount of light emitted upward from the liquid crystal panel 5 by controlling a voltage applied between the common electrode 34 and both of the transparent electrode 21 and the reflective electrode 20. In addition, it is made possible to display a desired image on the liquid crystal panel 5 by controlling the amount of emitted light for every picture element.
Incidentally, in the transflective liquid crystal display device having a structure shown in FIGS. 1 to 3, while light passes through the liquid crystal layer 40 only one time in the transmissive region, light passes through the liquid crystal layer 40 two times (to and fro) in the reflective region. Accordingly, there arises difference between the lights passing through the transmissive region and the reflective region as to the variances in the polarizing direction, and therefore the amount of light emitted upward from the panel unfavorably differs between the regions even if the same amount of lights enter the transmissive region and the reflective region. For this reason, even when a voltage applied is appropriately set for the device to exhibit an excellent display performance when the device is used as a transmissive liquid crystal display device, for example, excellent display cannot be achieved if the liquid crystal display device is used as a reflective liquid crystal display device.
There has been proposed a technique in which a thick insulating film is formed under the reflective electrode such that the cell thickness in the reflective region becomes half the cell thickness in the transmissive region. However, there arises a problem that the manufacturing cost is increased because of an increase in the number of manufacturing processes.
Japanese Patent Application Laid-Open Disclosure No. 2000-147455 proposes a transflective liquid crystal display device which changes the grayscale level depending on the on/off state of a backlight (light source). However, in the transflective liquid crystal display device disclosed in Japanese Patent Application Laid-Open Disclosure No. 2000-147455, the same voltage is always applied to a transparent electrode and to a reflective electrode, and accordingly the display quality (chromaticity among others) of images is deteriorated depending on conditions.
In addition, Japanese Patent Application Laid-Open Disclosure No. 2002-333870 discloses a liquid crystal display device in which one picture element region is divided into a plurality of sub-picture element regions, and in which a TFT and a transparent electrode are formed in each sub-picture element region. However, this liquid crystal display device is one which displays grayscale on the basis of digital image signals without employing a digital/analog conversion circuit, and therefore cannot be applied to a transflective liquid crystal display device.